This invention relates to compressors, and more specifically to an anti-stall casing treatment arrangement for turbo-compressors.
Turbo-compressors of the type used in aero-engines, industrial gas turbines, gas compression systems and pumps all have an aerodynamic limit of stable operation. Beyond this limit, a condition known as rotating stall occurs in which the smooth flow of gas through the compressor is disturbed by a rapidly rotating annulus of pressurised gas about the tips of one of more stages of the compressor blades. Where a complete breakdown of flow occurs through all stages of the compressor so as to stall all stages of the blades, the compressor will surge.
Turbo-compressors generally are designed to have a safety margin between the airflow and pressure ratio for normal operation and the airflow and pressure ratio at which stall will occur. It is desirable to raise the stall line to a higher pressure ratio for a given engine operation because this allows for an increase in the stall margin and/or an increase in the operating pressure ratio, and hence the performance, of the compressor.
Significant improvements in stall margin can be achieved by treating the compressor casing adjacent the tips of the compressor rotor blades. However, in conventional anti-stall casing treatment arrangements, which usually include slots, chambers and grooves in the compressor casing, improvements in the stall margin often are associated with a loss of compressor efficiency and mass flow at high speeds.
A known casing treatment is disclosed in a paper from The School of Mechanical Engineering, Cranfield Institute of Technology in Great Britain entitled “Application of Recess Vaned Casing Treatment to Axial Flow Compressors”, February 1998, A. R. Aziman et al; in an ASME paper in The Journal of Fluid Engineering Vol. 109, May 1987, entitled “Improvement of Unstable Characteristics of an Axial Flow Fan by Air-Separator Equipment”, Y. Mijake et al; and in U.S. Pat. No. 3,189,260. These publications disclose a mechanism including a recess for collecting rotating stall cells in post-stall operation. Since rotating stall extends a significant distance upstream of the rotor blades, it would appear that the recess has to be relatively large in order to be effective. While this kind of casing treatment is suitable for low-speed applications such as, for example, industrial fans and compressors, it is not suitable for aircraft applications where weight and space restrictions do not allow for a relatively large recess in the outer casing at the inlet of the engine or in front of a compressor.
A further casing treatment is disclosed in U.S. Pat. No. 5,762,470. This patent describes an annular chamber in the casing adjacent the tips of the rotor blades which communicates with the main flow passage in the compressor via a series of circumferentially spaced-apart slots. In use, pressure differences between the main flow passage and the annular chamber cause air to flow through the slots disposed about the rotor blades into the annular chamber and back into the flow path upstream of the rotor blades. A disadvantage associated with this particular type of casing treatment is that it requires a special coating on the ribs between the slots to protect these ribs from damage during blade contact. Since the width of the ribs and slots often is too small for adequate coating adhesion, the coating tends to fall away during compressor operation. On the other hand, if the coating is not applied, it is necessary to increase the tip gap significantly to prevent tip rub during operation, and this adversely affects the efficiency of the compressor. A further drawback associated with this type of casing treatment is that, for effective operation, it is necessary to have a relatively large annular chamber in the outer casing. As mentioned above, this is problematic for certain applications such as aero-engine compressors. Also, the relatively thin ribs between the slots are sensitive to resonance caused by the interaction of the rotor blades with the ribs, and accordingly the application of this treatment is restricted.
U.S. Pat. No. 5,282,718 discloses casing treatment in the form of an annular inlet located in proximity to the trailing edges of compressor rotor blades and leading to a plurality of anti-swirl vanes which are circumferentially spaced apart within an annular cavity, and an annular outlet leading back to the main flow path at a region adjacent the leading edges of the rotor blades. In this design, flow which is on the verge of separating from the blade tips is sucked into the annular chamber via the inlet and passes upstream through the antiswirl vanes primarily by means of the axial pressure gradient across the annular chamber. A drawback associated with this type of casing treatment is that, generally, the improvement in stall margin leads to a reduction in compressor efficiency and mass flow.
It is an object of the present invention to provide an alternative casing treatment for a compressor which is compact, relatively inexpensive to manufacture, and which improves the operating range of the compressor without adversely affecting the efficiency of the compressor.
For the purposes of this specification, the term “axial” refers to a direction parallel to the longitudinal axis of the compressor casing, the term “cross-sectional” refers to a direction perpendicular to the longitudinal axis of the compressor casing, and the term “radial” refers to a direction extending radially from or towards the longitudinal axis of the compressor casing.